We report on the environmental dependence of galaxy properties at z=2.15.We construct multi-band photometric data sets in the (proto-)cluster PKS1138-26field and in the GOODS field. We then fit spectral energy distributions ofthe galaxies with model templates generated with the latest stellar populationsynthesis code and derive physical properties of galaxies from the fits.To quantify the environmental dependence of galaxy properties, a specialcare is taken of systematic errors -we use data sets that have almost the same wavelength samplings,use the same code to fit SEDs with the same set of templates, and comparerelative differences between the two samples.We find that the PKS1138 galaxies have similar ages, shorter star formationtime scales, lower star formation rates, and weaker dust extinctioncompared to the GOODS galaxies at .This trend is similar to that observed locally, suggesting thatthe environmental dependence of galaxy properties isalready partly in place as early as z=2.15.We show that the PKS1138 galaxies assemblethe bulk of their masses 1Gyr earlier than field galaxies, i.e.,the galaxy formation depends on environment.Galaxy mergers should frequently occur during the first collapse of clustersand they might play an important role in driving the observed environmentaldependence of galaxy properties at z=2.15.Key words: galaxies: clusters: individual: PKS1138-26 - galaxies: formation - galaxies: fundamental parameters
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